As a method for electrically bonding first and second bonding target members with high bonding strength, ring mash (trademark) bonding, for example, is known (see Patent Document 1). This electrical bonding method includes providing a first bonding target member having a bonding target portion and a second bonding target member having a hole portion with a bonding target portion having an inside diameter which is slightly smaller than the outside diameter of the bonding target portion of the first bonding target member, positioning and placing the first bonding target member on the second bonding target member such that the bonding target portion of the first bonding target member slightly overlaps the bonding target portion of the hole portion of the second bonding target member, and applying a current, in this state, to the first and second bonding target members under pressure until their bonding target portions undergo plastic flow and pressing the bonding target portion of the first bonding target member into the bonding target portion of the hole portion of the second bonding target member to electrically bond the first and second bonding target members.
A feature of this ring mash bonding is that because the bonding target portion of the first bonding target member and the bonding target portion of the second bonding target member increase their bonding areas while undergoing plastic flow by the effect of the pressing force and bonding current and a vicinity of an outer peripheral surface of the bonding target portion of the first bonding target member and a vicinity of an inner peripheral surface of the bonding target portion of the second bonding target member are eventually solid-phase bonded to each other with a certain bonding width, high bonding strength can be achieved without being significantly affected by the dirt on the surfaces or the roughness of the surfaces of the bonding target portions.
When the bonding target members are made of a ferrous material containing carbon, such as carbon steel containing carbon above a certain level, steel having a carburized surface or cast iron, undesired quenching occurs during electrical bonding. Thus, even when high bonding strength is achieved by a ring mash bonding method, a bonded part including and in the vicinity of the bonded portion suffer the problem of becoming harder and more brittle, and, consequently, having lower mechanical strength. Therefore, when the bonding target members are made of a ferrous material containing carbon, such as carbon steel containing carbon above a certain level, steel having a carburized surface or cast iron, the bonded part needs to be tempered by applying a heating current to it after electrical bonding (see Patent Document 2).